1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
A technique for forming a transistor by using a semiconductor film formed over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit (IC) and an image display device (display device). A silicon-based semiconductor film is known as a semiconductor film applicable to the transistor. In recent years, an oxide semiconductor film has attracted attention.
For example, a transistor using an amorphous oxide semiconductor film which contains indium, gallium, and zinc and has an electron carrier concentration of lower than 1018/cm3 is disclosed (see Patent Document 1).
Since the oxide semiconductor film has a high electron mobility, the operation speed of a transistor using the oxide semiconductor film is significantly higher than that of a transistor using an amorphous silicon film. Further, the transistor using the oxide semiconductor film has an advantage of suppression of equipment investment, because production equipment for the transistor using the amorphous silicon film can be applied thereto with partly improvement.
Further, a memory to which characteristics of a transistor using an oxide semiconductor film is applied has been proposed (see Patent Document 2). Note that an oxide semiconductor film can be formed by a technique for forming a thin film such as a sputtering method. Further, a transistor using an oxide semiconductor film can be manufactured through a process performed at a low temperature lower than or equal to about 350° C. Therefore, there are few limitations on manufacturing a transistor using an oxide semiconductor film so as to overlap with a transistor, which allows reduction in the cell area.
In a transistor using an oxide semiconductor film, carriers may be generated due to an impurity such as hydrogen and oxygen vacancy, and the threshold voltage (Vth) of the transistor may be shifted in a negative direction. Therefore, controlling the threshold voltage (Vth) of the transistor using an oxide semiconductor film by employing a structure of a transistor including gate electrodes above and below its oxide semiconductor film, in which a back gate electrode which forms a pair with a front gate electrode is provided, has been considered (see Patent Document 3).